This invention relates to the field of AM and FM receivers and more particularly to those receivers which are capable of receiving both AM stereo and FM stereo. The present invention addresses a method of minimizing the components used to create this system and therefore the cost and size of implementing said system. This minimization of size and cost is necessary for AM Stereo to be enjoyed by the maximum amount of radio listeners.
In March of 1982 the FCC adopted its unique Report and Order authorizing the so-called "market-place approach" to the selection of a standard for broadcast of stereophonic program material in the AM band. At the beginning of the marketplace competition there were five competing systems. Over the past eight years the number of competitors has been reduced to two. Of these the C-Quam.RTM. system enjoys a preponderant advantage in the number of stations and has recently been declared the De Facto standard by the FCC. Over the period of this competition there has been significant research and development in attempting to build AM Stereo decoders that were capable of recognizing the different stereo broadcasting systems and decoding them. Several of these approaches were supplied by IC manufactures to various receiver manufactures in hopes that they would use them in their radios. However over the resulting years very few receivers have used these IC's. The perceived major cause for the very limited usage of these circuits is their expensive cost and relatively poor performance when compared to the single system IC designed for the Motorola system. The development of this single system IC was completed by Motorola in 1982 and over 20 million parts were sold by the 4th quarter of 1990. However, it has been observed by the inventor and others that this part is used almost totally by the automotive companies in their premium radio lines. This deprived the average listener of the pleasure and distant coverage benefits of AM Stereo.
The inventor determined that the best way to allow the benefits of AM Stereo to be enjoyed by a greater number of people was to reduce the cost without significantly lowering the performance of the receiver. Since the basic environment of the AM stereo receiver generally includes the FM stereo receiver, it was concluded that the best way to perform this goal was to design the AM stereo system to reuse as much of the FM stereo circuitry as possible. The present invention covers the switching and partitioning of circuitry to accomplish this goal.